A fuel cell is an electrochemical system in which a fuel (such as hydrogen) is reacted with an oxidant (such as oxygen) at high temperature to generate electricity. One type of fuel cell is the solid oxide fuel cell (SOFC). The basic components of a SOFC may include an anode, a cathode, an electrolyte, and an interconnect. The fuel may be supplied to the anode, and the oxidant may be supplied to the cathode of the fuel cell. At the cathode, electrons ionize the oxidant. The electrolyte may comprise a material that allows the ionized oxidant to pass therethrough to the anode while simultaneously being impervious to the fluid fuel and oxidant. At the anode, the fuel is combined with the ionized oxidant in a reaction that releases electrons that are conducted back to the cathode through an external circuit or to the cathode of an adjacent SOFC via the interconnect. Heat, generated from ohmic losses, is removed from the fuel cell by either the anode or cathode exhaust or is radiated to the environment.
A SOFC may be structured, e.g., as a segment-in-series or in-plane series arrangement of individual cells. The oxidant is typically introduced at one end of the series of cells and flows over the remaining cells until reaching the cathode exhaust outlet. Each fuel cell transfers heat into the oxidant thereby raising its temperature, and forming a temperature gradient which increases from the oxidant inlet to the exhaust. A temperature gradient may also develop in the fuel cell which increases from the oxidant inlet to the oxidant exhaust. These temperature gradients cause thermal stresses that may cause material degradation or failure of the fuel cell components, or may reduce fuel cell performance.
The anode of a SOFC may be a mixed cermet comprising nickel and zirconia (such as, e.g., yttria stabilized zirconia (YSZ)) or nickel and ceria (such as, e.g., gadolinia dope ceria (GDC)). Nickel, and other materials, may function not only to support the chemical reaction between the fuel and the ionized oxidant but may have catalytic properties which allow the anode to reform a hydrocarbon fuel within the fuel cell. One method of reforming the hydrocarbon fuel is steam reforming of methane (CH4), an endothermic reaction (Equation 1):CH4+H2O→CO+3H2ΔHº=206.2 kJ/mole  (Equation 1)
The heat necessary for methane steam reforming could be supplied directly from the heat released within the stack from the ohmic losses. This direct heat transfer helps to cool the stack, reducing thermal stresses and improving overall stack performance. However, in-stack reforming introduces several technical challenges. The unreformed methane must be supplied in the correct amount to avoid excessive cooling of the fuel cell and in the correct manner to avoid localized cooling. Additionally, hydrocarbon fuels have a propensity to form carbon, particularly when a significant amount of reforming is performed (Equation 2):CxH2x+2→C+(x+1)H2  (Equation 2)Carbon formation can cause fouling and degradation of fuel cell components through anode delamination, metal dusting and other failure mechanisms.
Consequently, supplying a mixture of a syngas reformed external to the fuel cell and an unreformed fuel to the anode may provide a better balance of system performance and durability than supplying either reformate or unreformed fuel alone. However, the ratio of reformed and unreformed fuel must be precisely controlled. If the ratio is too high, the large temperature gradient across the fuel stack will remain. If it is too low, carbon formation will result in loss of performance.
There remains a need for precise control of the ratio of reformed and unreformed fuels delivered to a fuel cell stack to ensure that the proper amount of reforming occurs internally to the fuel cell.